Bias current compensation device and method

Abstract

A compensation device that can include a bias-able device, a bias circuit that provides the bias-able device with a bias current, a signal conditioner selectively coupled to the bias-able device, and an emulator. The signal conditioner and emulator can divert current from the bias-able device in an operational and calibration mode, respectively. In calibration mode, the emulator generates a compensation current that is combined with a sense current so that the sense current equals the bias current.

1. A compensation device, comprising:
 a bias-able device; 
 a bias circuit that provides the bias-able device with a bias current, the bias current being a sense current less a first current; 
 a signal conditioner selectively coupled to the bias-able device that, when in an operational mode, is coupled to the bias-able device and diverts the first current from the bias-able device; and 
 an emulator that is selectively coupled to the bias-able device that, when in a calibration mode, is coupled to the bias-able device and diverts the first current from the bias-able device, the emulator generates a compensation current that is combined with the sense current so that the sense current equals the bias current. 
 
   
   
     2. The compensation device according to  claim 1 , wherein the bias circuit includes a variable resistance electrically coupled in series with the bias-able device. 
   
   
     3. The compensation device according to  claim 2 , wherein the variable resistance changes to adjust the bias current supply to the bias-able device. 
   
   
     4. The compensation device according to  claim 3 , wherein the variable resistance decreases to adjust the bias current until the sense current substantially equals a predetermined current. 
   
   
     5. The compensation device according to  claim 4 , wherein the sense current substantially equals the predetermined current when a voltage across a sense resistance substantially equals a reference voltage. 
   
   
     6. The compensation device according to  claim 2 , the variable resistance including:
 a first variable resistance coupled in series to a first terminal of the bias-able device that passes the sense current and the first current; and 
 a second variable resistance coupled in series to a second terminal of the bias-able device that passes current less than the sense current. 
 
   
   
     7. The compensation device according to  claim 1 , further comprising:
 a variable resistance that is coupled in series with the bias-able device; and 
 a state adjuster that monitors the sense current and changes the variable resistance to vary the bias current based on the sense current. 
 
   
   
     8. The compensation device according to  claim 7 , further comprising:
 a comparator that is coupled to the state adjuster, the comparator determines whether the sense current is substantially equal to a predetermined current. 
 
   
   
     9. The compensation device according to  claim 8 , wherein the sense current substantially equals the predetermined current when a voltage across a sense resistance caused by the sense current passing therethrough substantially equals a reference voltage. 
   
   
     10. The compensation device according to  claim 7 , wherein the state adjuster further comprises:
 a decoder that receives a reference voltage and controls generation of an initial resistance of the variable resistance; and 
 a counter that decreases the resistance of the variable resistance from the initial resistance until a comparator determines that the sense current is substantially equal to a predetermined current. 
 
   
   
     11. The compensation device according to  claim 10 , wherein the counter is coupled to the variable resistance so as to control the variable resistance, and thereby control the bias current passing through the bias-able device. 
   
   
     12. The compensation device according to  claim 7 , the variable resistance including:
 a first variable resistance coupled in series to a first terminal of the bias-able device, the first variable resistance passes both the sense current and the first current; and 
 a second variable resistance coupled in series to a second terminal of the bias-able device, the second variable resistance passes a residual current less than the sense current. 
 
   
   
     13. The compensation device according to  claim 1 , further comprising:
 a first offset current source that adds a first offset current to the first current, so that the compensation current includes both the first current and first offset current; and 
 a second offset current source that generates a second offset current, the second offset current being substantially a same magnitude as the first offset current and being subtracted from the sense current and the compensation current so as to substantially cancel the first offset current. 
 
   
   
     14. The compensation device according to  claim 1 , further comprising:
 a first switch that is open during operational mode so as to disconnect the emulator from the bias-able device, and closed during calibration mode so as to connect the emulator to the bias-able device diverts the first current from the signal conditioner to the emulator; and 
 a second switch that is closed during operational mode so as to connect the signal conditioner to the bias-able device, and open during calibration mode so as to disconnect the signal conditioner from the bias-able device. 
 
   
   
     15. The compensation device according to  claim 14 , wherein the first and second switches cooperate to direct the first current to the signal conditioner during operational mode and to the emulator during calibration mode. 
   
   
     16. The compensation device according to  claim 14 , further comprising a third switch that is open during operational mode to disconnect an offset current source, and closed during calibration mode to connect the offset current source, and thereby combine an offset current with the sense current. 
   
   
     17. The compensation device according to  claim 16 , further comprising:
 a switch controller that is coupled to the first, second, and third switches that is configured to open and close the switches appropriately in accordance with either the operational or calibration mode. 
 
   
   
     18. The compensation device according to  claim 1 , wherein the signal conditioner comprises:
 a differential amplifier that is coupled to the emulator to provide the emulator with a characteristic voltage of the differential amplifier. 
 
   
   
     19. The compensation device according to  claim 18 , wherein the characteristic voltage is an output node voltage that is coupled to a feedback resistance through which the first current passes in the operational mode. 
   
   
     20. The compensation device according to  claim 19 , wherein the characteristic voltage is supplied to an equivalent resistance of the emulator through which the first current passes in the calibration mode. 
   
   
     21. The compensation device according to  claim 1 , wherein the signal conditioner comprises:
 an amplifier that is coupled to the emulator to provide the emulator with a characteristic voltage of the amplifier. 
 
   
   
     22. The compensation device according to  claim 21 , wherein the characteristic voltage is an output node voltage that is coupled to a feedback resistance through which the first current passes in the operational mode. 
   
   
     23. The compensation device according to  claim 22 , wherein the characteristic voltage is supplied to an equivalent resistance of the emulator through which the first current passes in the calibration mode. 
   
   
     24. The compensation device according to  claim 23 , wherein the resistance of the equivalent resistance of the emulator is substantially equal to the resistance of the feedback resistance. 
   
   
     25. The compensation device according to  claim 1 , wherein the bias-able device is at least one of a magnetoresistor, a spin-valve, a photo-diode, and a thermistor. 
   
   
     26. A compensation method, comprising:
 biasing a bias-able device with a bias current, the bias current being a sense current less a first current; 
 in a calibration mode:
 diverting the first current from the bias-able device to an emulator that emulates a signal conditioner; 
 generating a compensation current that is based on the first current; and 
 combining the compensation current with the sense current so that the sense current equals the bias current; and 
 
 in an operational mode:
 diverting the first current from the bias-able device to the signal conditioner. 
 
 
   
   
     27. The compensation method according to  claim 26 , wherein the biasing the bias-able device further includes adjusting a variable resistance electrically coupled in series with the bias-able device. 
   
   
     28. The compensation method according to  claim 27 , wherein decreasing the variable resistance increases the bias current to the bias-able device. 
   
   
     29. The compensation method according to  claim 28 , decreasing the variable resistance to adjust the bias current until the sense current substantially equals a predetermined current. 
   
   
     30. The compensation method according to  claim 29 , wherein when a voltage across a sense resistance substantially equals a reference voltage, the bias current substantially equals the predetermined current. 
   
   
     31. The compensation method according to  claim 27 , wherein biasing the bias-able device includes:
 coupling a first variable resistance in series to a first terminal of the bias-able device that passes the sense current and the first current; and 
 coupling a second variable resistance in series to a second terminal of the bias-able device that passes a residual current less than the sense current. 
 
   
   
     32. The compensation method according to  claim 26 , further comprising:
 coupling a variable resistance in series with the bias-able device; 
 monitoring the sense current; and 
 adjusting the variable resistance to vary the bias current based on the sense current. 
 
   
   
     33. The compensation method according to  claim 32 , further comprising:
 comparing whether the sense current is substantially equal to a predetermined current. 
 
   
   
     34. The compensation method according to  claim 32 , wherein the adjusting the variable resistance further comprises:
 decoding a reference voltage and generating an initial resistance of the variable resistance based on the decoded reference voltage; and 
 clocking a counter that decreases the resistance of the variable resistance from the initial resistance until a comparator determines that the sense current is substantially equal to a predetermined current. 
 
   
   
     35. The compensation method according to  claim 34 , wherein clocking the counter adjusts the variable resistance, and thereby controls the bias current passing through the bias-able device. 
   
   
     36. The compensation method according to  claim 35 , wherein clocking the counter decreases the resistance of the variable resistance from the initial resistance. 
   
   
     37. The compensation method according to  claim 32 , wherein adjusting the variable resistance includes:
 coupling a first variable resistance in series to a first terminal of the bias-able device, the first variable resistance passing both the sense current and the first current; and 
 coupling a second variable resistance in series to a second terminal of the bias-able device, the second variable resistance passing a residual current less than the sense current. 
 
   
   
     38. The compensation method according to  claim 26 , further comprising:
 adding a first offset current to the first current, so that the compensation current includes both the first current and first offset current; and 
 generating a second offset current, the second offset current being substantially a same magnitude as the first offset current; and 
 subtracting the second offset current from the sense current and the compensation current so as to substantially cancel the first offset current. 
 
   
   
     39. The compensation method according to  claim 38 , wherein subtracting the second offset current diverts the second offset current from the sense current and the compensation current. 
   
   
     40. The compensation method according to  claim 26 , further comprising:
 opening a first switch during operational mode so as to disconnect the emulator from the bias-able device; 
 closing the first switch during calibration mode so as to connect the emulator to the bias-able device diverts the first current from the signal conditioner to the emulator; and 
 closing a second switch during operational mode so as to connect the signal conditioner to the bias-able device; and 
 opening the second switch during calibration mode so as to disconnect the signal conditioner from the bias-able device. 
 
   
   
     41. The compensation method according to  claim 40 , wherein
 opening the first switch and closing the second switch directs the first current to the signal conditioner during operational mode; and 
 closing the first switch and opening the second switch directs the first current to the emulator during calibration mode. 
 
   
   
     42. The compensation method according to  claim 40 , further comprising;
 opening a third switch during operational mode to disconnect a source of an offset current; and 
 closing the third switch during calibration mode to connect the source of the offset current and thereby subtracting the offset current from the sense current and the compensation current. 
 
   
   
     43. The compensation method according to  claim 26 , further comprising:
 coupling a differential amplifier to the emulator to provide the emulator with a characteristic voltage of the differential amplifier. 
 
   
   
     44. The compensation method according to  claim 43 , wherein the characteristic voltage is an output node voltage that is coupled to a feedback resistance through which the first current passes in the operational mode. 
   
   
     45. The compensation method according to  claim 44 , wherein the characteristic voltage is supplied to an equivalent resistance of the emulator through which the first current passes in the calibration mode. 
   
   
     46. The compensation method according to  claim 26 , further comprising:
 coupling an amplifier to the emulator to provide the emulator with a characteristic voltage of the amplifier. 
 
   
   
     47. The compensation method according to  claim 46 , wherein the characteristic voltage is an output node voltage that is coupled to a feedback resistance through which the first current passes in the operational mode. 
   
   
     48. The compensation method according to  claim 47 , wherein the characteristic voltage is supplied to an equivalent resistance of the emulator through which the first current passes in the calibration mode.